Frequency modulation monitor



Feb. 11, 1947.

J. M. ABRUMBAUGH FREQUENCY MODULATION MONITOR Feb. ll, 1947.

J. M. BRUMBAUGH 2,415,456 v FREQUENCY MODULATION MONITOR Filed June 19, 1942 INVENTOR c/abw/Ma 6,400#

BY W

ATTORNEY Feb. l1, 1947.

J..M. BRUMBAUGH 2,415,456-

FREQUENCY MODULATION MONITOR Filed June 19, 1942 Y, n Sheets-Sheet I5 INVENTOR domv M? f4/M9406.

ATTORNEY Feb. 11, 1947. 1 M. BRUMBAUGH 15,4

FREQUENCY MODULATION MONITOR Filed June 19, 1942 5 Sheets-Sheet 5 E. c.

FMS/MRI A TIO/V INVNTOR ATTORNEY Patented Feb. 11, 1947 2,415,456 FREQUENCY MODULATION MONITOR lohn M. Brumbaugh, Lansdowne, Pa., `assignor to Radio Corporation of America, a corporation of Delaware Application .l une 19, 1942, Serial N0. 447,629v

Claims. l

This application concerns a new and improved modulation analyzer or monitor which provides continuous visual indication of the modulation level.

An object of my invention is to provide a practical instrument for demodulation of frequency modulation signals with relatively little distortion and for measurement of modulation per cent of a frequency modulation signal and to provide a direct current voltage component proportional to deviation of the mean carrier frequency.

In my monitor a panel meter indicates modulation level peaks in accordance with the standard time constant set up .by thc Federal Commum'cations Commission and a flasher lamp indicates audio peaks which exceed a pre-determined level. By means of a novel audio ampliiier including a phase inverter, the flasher lamp is caused to indicate negative and positive audio peaks. The audio amplitude is proportional to the deviation of the carrier. The flasher circuit also operates a relay which may control the operation of an external alarm or a peak counter. An external meter may be connected from a control desk or other remote location to the instrument for modulation indication. The system includes a demodulator having a low distortion characteristic which may be used forr distortion measurements on the transmitted wave or on the overall transmitting system.

My modulation monitor also serves as a frequency modulation demodulator of high quality having low distortion characteristic and it may be used for distortion measurements on the transmitter or the over-al1 transmitting system.

Provision is made for the attachment to my system of a carrier frequency monitor or meter. Attachment of this frequency monitor or meter and application to the modulation monitor of theV additional circuits and energy to operate the carrier frequency monitor in no way aiects the operation of the system as a modulation monitor.

My system has many advantages over monitors known in the art today and is, as a matter of fact, as far as I know, the only complete modulation monitor of this type satisfying the requirements of the Federal Communications Commission and including a distortionless modulation demodulator and ampliiier known in the art today.

In describing my invention reference will be made to the attached drawings wherein Figure 1 shows schematically by block diagram in simpl-ifled form my modulation monitor; while Figures 2 to 2c inclusive illustrate in considerable detail the circuit connections of a modulation monitor arranged in accordance with my invention. The monitor Ainput is in Figure 2 and the circuits progress tothe right through Figures 2, 2a, 2b and 2c.

Referring to Figure 1, the frequency modulated wave is supplied to a connection marked R. F. Input and thence to a heterodyning or mixing stage in a unit included in rectangle i. In this mixer stage the modulated energy is heterodyned against oscillations of substantially constant frequency supplied by a crystal controlled oscillator in unit 3 after multiplication in a frequency multiplier 5 here shown as a frequency tripler. For purposes of example, the oscillations out of il are assumed to differ from the radio frequency carrier by minus 200 kc. The difference frequency is supplied from the output of mixer i to a radio frequency filter in unit l, and in the example given, this output intermediate frequency is of 200 kc. plus and minus the modulation swing. After filtering in unit l, the modulated I.F. is supplied to an I.F. amplifier in unit 9 and `therein additionally filtered and amplified. The 'frequency modulated wave out of unit 9 may vary in amplitude because of the effect thereon `of the pass characteristics of filters l and `9 or'ior other reasons. Amplified intermediate frequency modulated energy is supplied from 9 to an intermediate frequency amplitude limiter and square Wave former in unit l l and thence to a cycle counter or demodulator in unit i3. The

frequency modulated wave herein as applied to the frequency modulated wave detector is caused to have the .form shown above unit l i. The amplifier in 9 limits or clips the wave to remove therefrom the amplitude variations.

In this unit i3 by new and improved means the modulations on the carrier are detected and the output is supplied through a low pass filter in unit l5. The input l5 provided by the counter circuit is a uni-directional pulse of pre-set coulomb content for each I.-F. cycle. rIhe cycle counter includes a condenser charged by the square wave and its charge is built up in increments which are superposed on the uni-directional current. The low pass lter smooths out this direct current so that the pulses are averaged in the low .pass ilter in l5 to produce at point il Va steady direct current (no modulation) or a smoothed audio frequency wave form correspending to the 'modulation of the carrier impressed on the mixer in unit l and also the modulations on the `I.F. in .say the filter and amplifier in unit 9. The low pass iilter also removes the intermediate frequency components. This steady direct current or smoothed audio frequency wave form is fed from the output of I to a high-class audio amplifier in unit I9 and also by way of a modulation range switch SSA to an audio amplifier in unit 23.

The audio amplifier in unit I9 may be coupled by a switch S IB to an additional amplier 21 and thence to an audio frequency output including, if desired, locally connected phones and, if desired, a line 3i to a remote point. The amplifier I9 also l includes a de-emphasis network to be cut in by moving switch SIB to the contact 29 when pre-` be connected by switch S3 to a point in theoutput of the phase reverser 23 at which a voltage which measures the positive peaks or a voltage which measures the negative peaks appears. Thus, by manipulating switch S3, one may produce in the ilasher 39 negative and positive peak modulation indications, that is, percentage modulation. The output of the flasher voltage amplier in 33 may also actuate a relay circuit to control a remotely located counter alarm. The peak indicating flasher circuit in unit 33 includes also percent modulation control circuits in the form of a resistive network in the cathode bias circuit of an amplier in unit 33 to regulate the gain thereof. This network cooperates with a voltage regulated source of supply current.

The output of the ampliiier including the phase reverser in 23 is also supplied by lines 31 and switch S4 to a peak indicating meter circuit 4I, including current amplifying tubes, one of which has a meter in its cathode return circuit. The output of the circuits in 4I may be also supplied to a peak indication of percent modulation meter 43.

The amplier in 2.3 also includes an audio frequency gain adjustment circuit in the form of an adjustable feed-back networkV for the initial amplier tube by means of which the audio gain is standardized. By this means the gain Aof the amplifier may be readjusted at any time; This is for the purpose of relating the amplier system gain with the strength of the modulated wave derived from the mixer as recorded by the meter.

The meter i3 may also be connected by switch S9 and switch StB to check up on the strength of the mixer stage (unit I) output and the output of the crystal source in unit 5 as supplied to the mixer.

The output of amplier 9 is also supplied to a rectifier in unit 41 to derive automatic gain control potentials for the tubes in unit I. This AVC current may also be supplied, after amplification, through a radio frequency metering circuit in unit 5I and thence by switches SBA and S'B and S9 to the panel meter 43. The oscillator output, that is, the output of the circuits in 5, can be checked by passing the direct current supplied to the final stage therein by lead 5'! through 2,415,456 i I V switches SBA and SBB and S9 to this panel meter 43.

A voltage regulator for the wave former stages in unit I'I is included in unit 6I, which also includes means for adjusting the amplitude of the input to the frequency modulation detector or counter circuits .in unit I3. Other refinements, such as a voltage regulator for the amplifier in unit 23, are used throughout the circuits as will appear more in detail hereinafter.

The peak-to-peak input of the discriminator is checked by connecting the output of II through lead 63 and switches SSA and SSB and A S9 to the panel meter 43.

Referring to Figure 2, the frequency modulated energy to be analyzed is picked up on loop 2, by induction or by coupling the same to a line passing frequency modulated waves, and fed to a control electrode IB of a converter tube VI. A conventional crystal control oscillation generator, including tube Vit, crystal A2 and anode tank circuit L1, and C54 supplies oscillations of constant frequency to a tripler stage including tube VH and tuned tank circuit C13-L8. The mixer1 stage including Vl is included in rectangle I of Figure 1, while the generator and frequency multipliers including tubes VIG and Vil are included in units 3 and 5 of Figure 1. This oscillation generator and tripler are coupled cadenser C8 pacitively rather than by transformer, and since the operation thereof will be obvious from the showing thereof, they will not be described in detail herein. It will be noted, however, that a tuning indicator may be connected to jack J4 for tuning and other adjustment purposes, so that at the output tank circuit C10-L8 oscillations of the proper frequency and amplitude are supplied by inductance L9 and lead I4 to the control grid I6 of tube VI to heterodyne therein with the received wave.

The intermediate frequency is supplied by a filter circuit comprising resistive and inductive couplings Rit-LI, resistor R8 and coupling conto the control grid 20 of an intermediate frequency amplifier stage V2. This lter is included in the unit 'I of Figure l. The amplified intermediate frequency is supplied from the anode 4i! of V2 by way of a coupling condenser CIB and resistance RIS to the grid 4d of a tube V4, (Figure 2a), which comprises the iirst stage of an amplitude limiter and square wave former including tubes V4 and V5. These tubes are included in rectangle II of Figure 1, along with, if desired, the peak-to-peak meter tube V6.

The output of tube V2 is also supplied to a double purpose tube V3 wherein the I.F. is rectified in the diode section to derive between the resistance R22 and ground and on lead 48 automatic volume control potentials which are supplied back to the grid I of tube VI for gain control purposes. This gain control circuit is included in unit i of Figure 1. The feed-back potential is also supplied by line 49 to the grid of a triode in V3 wherein it is amplied and taken from the cathode resistance R25 and passed over line 5I and switches SSA and B to the meter I3 for radio frequency input readings.

The radio frequency wave as received may be of varying amplitude and/or the same may be modulated in amplitude while passing through the stage including tubes VI and V2 due to the bandpass characteristics of the circuits. This un.. desired amplitude modulation is removed substantially completely so that a frequency varying wave of ilat topped form is fed to the cycle counte'r (unit L3, Figure 1). To so. modify the wave thev stages V4. and V5. are coupled in cascade as shown and biased and operated to clip 01Tk the rounded portions of the wave to thereby limit the amplitude of the I. -F. signal and cut off the peaks thereof to produce an output signal at resistor R35, which is substantially of square wave form. The tubes V5 and V5 or one thereof may be considered biased to a gain such that saturation output is attained for allr usable input voltages. The anode S5 of V5 is supplied by direct current through R35 and lead E5. The amplitude of the square wave (as applied to the frequency modulation detector or cycle counter) may be adjusted, by means of resistor RI I3 which controls the output voltage of the voltage regulator tube VIS cooperating with the tube VIS and connected by lead .iii and lf3. (Figure 2b) with the B supply including tube V (Figure 2c), to a certain cali brated value, which is indicated on the panel meter 43 (Figure 2c) when` it is properly connected to the anode 6 of tube V5 through the tube V5 by means of lead @l and the input and input check switches Seli and S53 and S9. This circuit through tube V5 and the meter I3 is as follows: Switch 555A as shown is the oscillator check position. This switch is moved clockwise two positions, The path from the top of resistance R is through tube V5, R37, R38, lead t@ (see Figures 2b and 2c) switch SA, SSB and line il to voltage regulator tubes VIS and VIS. Now if switch S9 is pushed up, the meter d3 is included in this series circuit.

The condenser C25, diode dI and diode d2 of V'i and resistor RI@ provides a frequency modulation detector or counter circuit which counts the intermediate frequency cycles to provide an output proportional to the frequency of the intermediate frequency energy.

When a square wave voltage is applied to the input of tube V5, on negative swings the plate potential at anode 55 reaches its peak value at the same time C25 is charged by R36 through the diode di to a peak value with the end of condenser C25 adjacent di negative. Now when the input to tube V5 swings positive, the plate potential at BS and on R55 goes to its minimum value and C25 discharges through diode d2 in series with its load R52 and R53 at a rate determined by resistors RM and R43. The time constants are made such that the diode coupling capacitor C25 is completely charged or discharged during each half cycle of the square wave. As long as the voltage of square waveV form remains substantially of constant amplitude, the total quantity of electricity which flows through R42 in each cycle of the I.F is constant. Thus, an increase in the repetition rate or I.F. frequency increases the number of current pulses per unit of time and, therefore, increases the average value of current through R42, of this is that its response to the frequency deviations or changes is linear and is unaffected by large changes in the rise time and roundness of corners of the square wave; that is, the wave front per se has no effect on the linearity of the output of this detector, provided that the time constants are made such that the diode coupling capacitor C25 is completely charged or discharged during each half cycle of the I.F. square wave.

rThe direct current with superposed audio output in Rl results from intermittent excitation of the condenser C25 by the square wave I.F. and,vas a consequence, has superposed thereon snarp pulses.- To smooth'out these `sha-rp pulses,

The advantage I pass the output from resistor R42 through a low' The detector circuit associated with the tube V'I is of the counter type. which is inherently linear over its operating range since its linearity doesnot depend'V upon the alignment of tuned circuits. the capacitor C25- charges and discharges during one-half of an I.F, wave cycle and, therefore; a definite quantity of electricity flows in the resistor R52 in the form of a relative sharp pulse during each I.-F. wave cycle as shown by the curves on the block diagram in Figure 1. The average values of current flowing through this resistor follow the corresponding values of modulation level of the I.F. carrier. pulses are filtered out by the low pass filter IEM! following the diode V'i and the voltage appears through line I .I0 and across resistances R44 and R55 of a wave form similar to that of the modulating audio voltage at the transmitter. This voltage constitutes the audio .signal, The diodes V'I are included in unit I3 of Figure 1. and the low` pass filter |55. is included. in unit I5 of Figure l.

The smoothed current produces a voltage which is fed to Rfid` and. R45, Figure 2b, and thence to the range switch SSA which is set for a large or small. frequency deviation of the frequency modulated carrier being. monitored.

Thisvoltage is fed by switch SEA to the-grid |20 of the amplier tube V8r andfrom the anode |26 tothe grid. |30 of the tube V19 wherein it is' amplified and then fed to the control grid |66. of a phase reverser tube. Vil). Note that the couplings between tubes comprise resistances and condensers.

This phase reverser tube V I0 has its control grid |60 coupled by resistance R50 and resistance R52to its cathode |54, While its control grid i60 is alsocoupled by a relatively high resistor RSI to lead |68 connectedwith the positive potential source, which also supplies the anode of the tube VIU. The potential supplied to the anode I'Ilv and grid |60 of the phase inverting tube VID is derived by lead |68 from the main source including rectifier tube V25 (Figure 2c) the cathode of which is connected by filters and lead |13.and voltage regulator tubes V22 and V23 to lead. I68. Thus,rthe grid |60 is in effect connected to a resistance potentiometer across the plate supply source.

The applied modulation voltage is reversed in phase by the tube action so that the voltage on the anode. |10 is reversed with respect to the voltage applied to the grid |55. The voltage in R62, however, is inphase with the grid voltage. Thus, if vI take the output from the anode resistances, Rand R54, itis a measure of peak deviationsof theapplied` frequency modulated Wave in one direction.v If I. take the output from the cathoderesistance R62, it is a measure-of peakV deviation ofzthewave in the other direc-y e tionz... By.; noting the-phase:reversalsibetween the-V The circuit time constants are such that.

input of the' system and the contacts at the output lof tube VIII, the flasher switch S3 and the meter switch S4 contacts can be appropriately marked as shown with a or indicating that the frequency of the wave is above and below respectively the mean or carrier irequency.

The phase reversed outputs are taken across resistance R62 and from R63 and supplied by leads including coupling and direct current blocking networks |14 and |16 to the flasher switch S3 and meter switch S4 (Figure 2c). The purpose of this reverser tube VI is to permit negative as well as positive peaks of the modulation to be analyzed. In this reverser arrangement, as shown, the full cathode bias resistor R62 drop is supplied to the control grid |60 and then to compensate this high negative potential, some positive potential is supplied to the control grid by resistor RBI. Thus, the signal of reversed phase is obtained from balanced resistors R62 and R63 in the plate and cathode of the same tube, and the voltages must be identically equal in magnitude since they are produced by the same current lowing in matched resistors. Thus, the voltage balance is independent of tube gain. In arrangements known heretofore the bias has been obtained by means of a tap on the cathode resistor and this gives full cathode degeneration only if the grid is driven from an impedance which is quite low compared to the grid resistor. The bleeder bias method of the present invention gives full degeneration with a high impedance source. It also tends to maintain a more stable operating point on the tube characteristic. As is well known, many phase inverters of the prior art use two tubes and, as a consequence, are subject to changes of currents due to changes in the two tube characteristics. Here a fundamental requirement is that the impressed voltages give phase reversed` voltages identically equal in magnitude and the arrangement shown meets this requirement.

The phase reversed Signals are then supplied by switch S4 to a meter rectifier diode VII and from diode load R12 to amplier tube VIZ and appear in the cathode load R15 and R15A. If desired, a meter may replace the resistor-RHA, this meter being on a remote panel and similar in nature to meter 43.

These meters then indicate the signal peak maximums, both positive and negative when the switch S4 is moved and indicate the variations of the peak maximums. This assumes that S9 is the position shown to put RISA, or the remote meter replacing the same, meter Hand R15 all in the cathode circuit of tube Vl2. Note that the diode VI I is of the cathcdefollower type, the output therefrom being supplied from resistance R12 to the grid |82 of tube Vi2, the anode |84 of which is connected by a resistance potentiometer R14 and R15 to ground and by switch S9 through the meter 43 to the cathode resistance R15A. The plate |84 is connected to the main supply source including tube V25.

The output of the phase reverser tube VIII is also connected by switch S3 to a flasher contact and thence to the grid 230 oi a triode amplifier VI3, this amplifier also being of the cathode follower type and having its cathode load R18 coupled by an over load protecting resistor R80 and bypass condenser C53 to the controlling electrode 240 of a flasher gas tube VI4. The tube VI3 serves as a coupling and isolating amplifier for the asher and relay tube VI4 to prevent it 8 from reacting on the meter circuits and tube V|2. The bias of tube VI4 is adjusted by the graduated flasher control including resistor R8I and the resistor network comprising R82, R84,

R85, etc. coupled with the voltage regulator tube VI5 and by lead |13 to the main supply source.

This bias as controlled by the tap on resistor RBI determines the peak audio voltage and thus the percentage modulation at which the flasher AI connected with the anode of tube VI4 and the relay 260 will operate.

The glow tube AI is connected to the positive side of rectifier V24, the negative side of which is connected to the cathode of tube VI4. The tube AI is also shunted by RSU so that the gas tube V|4 is in a series circuit across the output of rectifier tube V24. 'Hits circuit all floats above ground as R8! is varied. The bias on grid 245 swings and adjustment on peaks is such that tube Vid conducts and the voltage across VI4 drops while the voltage across AI raises and AI discharges or iiashes.

The winding EI of relay 260 is in series with the cathode direct current circuit of the tube VI4 and with the independent power supply source ierluding rectifier V24 and high voltage winding When the flasher tube AI breaks down winding 2E@ opens normally closed contact 25| and closes contact 263 to close at 280 a circuit conneet-ed to any remotely located alarm or calling circuit or signal light arrangement. In the presence of sustained over-modulation the audio keeps tending to break down tube VI4. However, winding 250 opens contact 25| and the relay is de-'energized to let contact 26| close and the cycle is repeated. This gives an intermittent indication of over-modulation by ringing the bell or iiashing a light on and off at the remote point connected to the output of 280.

The flasher AI gives individual indications of peaks following each other by not less than .l2 second, which is the time required for recuperation of the flasher circuits. On sustained high modulation levels the flasher AI and relay 26B operate intermittently at the rate of about 8 cycles per second.

The potential drop at the cathode end of resistance R52 in the phase reverser stage tube tube Vid is also supplied through a network including resistance R61 to lead |21 and thence to a movable point on gain control resistance R45. The purpose of this automatic gain control will be described later.

The output of tube V1 appearing across resistors R42 and R43 is also supplied by lead H2 (Figure 2a) through coupling condenser C11 to the grid SBI oi a tube V26 (Figure 2b) of a low distortion amplifier circuit designated generally at 30g. This ampliiier circuit comprises two tubes, V20 and V2I, coupled in cascade by resistances and condensers to amplify linearly the smooth audio frequency supplied to the grid 33| oi the iirst tube V20 by coupling condenser C11. The output oi this amplifier, which corresponds to units I9 and 21 in Figure l, may be taken from the leads marked A. F. Output. The ampliiier may include in the coupling between stages a cie-emphasizing circuit including condenser C19 and resistor RI2I connected in parallel between the anode 335 of tube V20 and the upper terminal of the resistor RI22 in the plate supply circuit. By cutting in resistance RI I8 to include C11 inV shunt to RIZI, the voltages fed from weer .9` the anode 300 to the grid of tube V2 follow linearly the voltages supplied on the grid 30|. i

By placing the short circuit across resistance RMB' by means of switch SIB (in the position shown), C is directly in shunt to R|2i to deemphasize the modulation potentials as their frequency increases. The purpose of Rilti is to maintain the average potential drop across R|2| on C20 and prevent key shocks when switch StB is moved. The switch SEB corresponds to the switch Si B and contact 20 of Figure 1, while the stage V2! is the audio amplifier of unit 2'! of Figure 1.

In order to maintain insofar as possible constant potentials on the stages the oscillation generator and in the mixer and intermediate frequency amplier stages, I provide at 500 (Figure 2a) a Voltage regulator. This Voltage regulator is not being claimed herein and a detailed description thereof is believed undesirabley at this point.

A Voltage regulator is also shown at M (Figure 2b) for supplying potential to the modulation amplier stage comprising tubes V8, V9 and V50, substantially constant anode and screen electrode potentials. The main power supply comprising rectifier V25 supplies plate and screen electrode potentials directly or after regulation in the voltage regulators 400 and i 0.

If the voltage across resistors RM and R45, Figure 2b, that is, at the output of the demodulator or counter circuit varies as the intermediate frequency varies and the gain of the following audio frequency amplifier including tubes V0 and V9 is standardized, the peak audio-,frequency voltages appearing at the anode |80 of tube VH and, therefore, indicated on the panel meter 43 connected to R'I5A will be a direct indication of the peak modulation. The monitor is adjusted to fulfill these conditions during production test by adjustment of the bias resistor R43 for diode d2 and potentiometer resistance RISE in the audio frequency gain standardizer indicated genorally at 200.

This gain standardizer includes a source of audio in the form of .a secondary winding 20| shunted by a resistance network. A point on resistance R|S5 of this network is connected by lead |90 to a contactof switch SZA, Figure 2b, and thence to the grid |20 of the amplifier tube V8. A second point on this network at resistance R530 is connected by lead |02 to switch S5 and then to the anode 180 of diode tube V| l. As will appear more in detail hereinafter, the audio current is fed to the meter 43 through ampliers V0, V0, tubes W0, VI I and V|2, or through tubes Vil and V I2 by changing the position of .switch1 S5.

Assume that the system has been all set up in good working order including proper adjustment of the feedback, in the amplifiers V8, V9 and tube V|0, over line |27 and adjustable .gain control potentiometer R09 in the cathode circuit of tube V0.

Say the adjustment is complete and in the presence of an input wave which has a total 150 kc. deviation, we have lout of V1 or into V8 an alternating current or audio signal of fixed iritensity which represents this swing and isproper to operate the flasher circuits and meter circuits, and I do not want this current to `become more or less in the presence of a wave of the above nature. If this current .should vary due to ageing of tubes or for some other reason, the flasher tube AI might not :tias-h -on peaks and mal R. F.

the meter G3 would not be driven to 100% by a 150 kc;v input signalV deviation.

The amplifiers vV8 have had or now have their gain adjusted by regenerative `feedback lead |21 and potentiometer resistance Rit until modulation is indicatd in the meter 23 with 150 kc. input signal deviation, and the flasher AI operates when R8| is Yproperly set. Next we close switch .SZA onthe contact marke Test and put in audio from the network 200 (Figure 2c) and nip the switchE back and forth so the meter is fed alternately by audio potentials from line |00 ampliiiedA by tubes V0 to V 2 or by audio potentials from line |92 supplied to the meter through tubes V|| and V|2 only. Now adjust the ratio of the two supplies over lines |00 and l 02 until the meter 43 reading is the same in both positions of switch S5. This ratio is adjusted by the movable point on resistance R|35 of standardization network 20, and may be, say 1 to 74, so the gain in tubes V0, V5, V|0 and VH is 75.

Now if in the future we want to check up on the gain of amplifiers V0, V0, etc., we put audio from 200 on V3 .and manipulate the gain check switch S5. If the meter i3 readings are diiferent in the two switch positions, the feedback adjustment at potentiometer R40 is changed until the meter readings are the same in tions. Thus we know that if the monitor up to the counter circuit is responding properly, the flasher tube A| and meter i3 will give correct indications of the modulation peaks and percentage modulation. If they do not, we can check the gain of the amplifier by the gain standardizer circuit 200 and adjust its gain to a proper amount.

Operating calibrations are checked thereafter in terms of the Apanel meter i3 indications and adjustment made by operation of the feedback adjustment R40 for the audio amplifier V0 and V9 and by adjustment of the gain of tubes Vil and V5 by use of R||3 (Figure 2a) associated with the voltage regulator 00 to supply out of the cycle counter the proper steady direct current when the incoming wave deviates through a given range. Y

The accuracy of these check circuits is dependent upon primarily the stability of precision resistors and of the meterl circuit. The overall accuracy of indication is thus dependent upon the stability of capacitor C25 and-resistor R52 in the anode circuit Aof diode V1 and the check circuit mentioned above.

'I'he instrument is calibrated, in manufacture, according to the following procedure:

(a) The usual type of mechanical and electrical inspections are made; including circuit continuity check, voltage measurements, tuning, distortion and frequency response checks, etc.

(b) Calibrated 200 kc. signal is introduced at B in the grid circuit of V2, sufficient to give norindication in the AVC metering circuit. This metering circuit, as stated above, comprises the triode section of V3 excited by the drop through resistance R22 resistance R25, line Y5|, switches SEA, SSB and S0 vand meter 43. A 50,000 ohm rheostat is substituted for R50 in the cathode circuit of V'i, and this 50,000 ohm 4resistor and R41 in the input circuit of tube V8 (Fguregb) are varied to give: (a) zero potential on the direct current output of the cycle counter V7 (Figure 2a) and (b) minimum shift in this potential when the power line voltage isvaried. M41 is then left at this setting, and substitution both switch posithe Test position Y 11 is made for R40 onlywhen the instrument'is used with a frequency monitor, in which case this control is used for zero calibration of the meancarrier deviation scale.

(c) Calibrated intermediate frequency signals of 100, 200 and 300 kc. are supplied at the same input point, B. and R43 adjusted to give 1.02 volt change in direct current potential at direct current output jack, J3, Figure 2b and D in Figure 1, or 0.765 volt peak to peak for 150 kc. swing. During this operation, the detector input (peak to peak square wave voltage) is kept at its proper value, as shown by the meter indication on Discr input. As stated above tube V6, Figure 2a, and its circuits including R38, line 54, switches SBA and S9 complete a square wave peak to peak` indicating circuit through meter 43, Figure 2c. The linearity of response of demodulator is checked.-

(d) With the "Gain control,- R49, Figure 2b, set to give 100% meter indication when 0.2707 volt R. M. S. (0.765 volt peak to peak) of audio signal is applied to J3, the Gain switch 52A at on contact labeled Test, and SIA set at '75 kc. position, RI35 of the gain standardizer network, Figure 2c, is soadjusted'that the meter indication does not change when S5 is alternately pressed and released.

(e) R83 and the knob on RBI, Figure 2c, are set so that the iiasher lamp Ai, Figure 2c, 39 Figure 1, lights when the meter indication is equal to the setting of the "Flasher control (RBI) on its per cent modulation scale.

(f) Checks for elimination of effects of ampli tude modulation on signal, hum and noise level, operation of asher relay, etc.

After proper calibration, as outlined above, the normal operating procedure is as follows:

(a) A crystal is selected such that its tripled frequency will be 200 kc. below the transmitter mean-carrier frequency. Oscillator VI 6 and tripler VI'I are tuned to give proper meter indication for the Osc check operation. The Input switch, SSB (Figure 2c), is` turned to OSCJ and Check button, S9, is pressed. The meter circuit is from the grid I6 of tube VI, lead I4, RIM, RI 03, lead 63, Figures 2, 2a, 2b, switches SBA, SBB, S9 and meter 43.

(b) Sufficient transmitter signal, about 1 volt,`

is applied to the.R..F. Input at resistorV RI, Figure 2, to produce proper meter indication on the R. F. Checkoperation.

(c) The Demodulator input control, RI I3, is set to its proper position, as indicated on the meter by the .Demodulator Check operation.

(cl) The audio frequency Gain switch, SZA, Figure 2b, is vturned to Test control, R49, is set so that the meter deflection remains unchanged when the gain check button, 60

S5, is pressed and released.,

(e) The Kilocycle swing switch, SIA, Figure 2b, is set to the desired condition of sensitivity and rie-emphasis, the Flasher control, Figure 2c, at the desired warning level, and the Gain switch SZA on the contact as shown, that is, Ythe operating contact, to Open The meter will now indicate per cent modulation on the transmitter signal. of the polarity indicated by the Meter and Flasher switches, S4 andSB. i

It should be noted that the proper settings of the audio frequency, Gain and Demodulator input controls are not entirely independent of supply-line voltage, and they should be checked whena modulation reading of highaccuracy is desired. However, Vunder normal operating conand the fGain.

12 ditions, the adjustment controls will require very little re-setting. Y

The instrument also serves as a de-modulatcr of very low inherent distortion and may be used for system distortion checks, noise checks, etc.

Since many programs, particularly of speech, are quite unsymmetrical as to peak amplitudes, checks should be made at the beginning of programs for the presence and direction of such unbalanced peaks. The Meter switch is turned quickly from plus to minus and vice versa during modulation peaks, and is finally left on the polarity which shows the larger peaks. The Flasher should be set on the same polarity and the level control at the desired warning level.

Normal operating procedure should include daily checks of the various panel adjustments after a normal warm-up period.

When my system is to be used with a carrier frequency monitor, a constant frequency calibrated source supplies input'to the intermediate frequency amplifier by connections to the dotted contact B. The crystal in unit 3 is removed and the tubes therein are used as amplifiers and fre quency multipliers. The crystal in the frequency monitor is connected through dotted line labeled A to the stages in 5. A very stable oscillator is required in wave metering and the monitor has such an oscillator. The radio frequency input to mixer I is interrupted and the demodulator in I3 is adjusted for a known output at 200 kc., this frequency being derived by the calibrated input at B. This output is fed to a line G'I, Figure 1, and thence to a sensitive meter connected to D. A direct current which bucks this output is sup plied in line 6l from the monitor and adjusted to buck out or compensate the output of the discriminator until a zero center reading is had in the meter connected to D. The radio frequency input is now applied and, if correct, the demodulator output will be a mean average such as to compensate the current supplied at B1 and cause in the meter a zero reading indicating that the mean carrier is of a frequency differing from the frequency in the output of 5 by exactly 200 kilocycles. Deviations from this proper mean frequency will result in changes in the direct current output. in line SI, which will cause the meter hand to deviate from zero in a direction depending on whether the mean carrier is too high or too low.

In order to minimize distortion at the higher audio frequencies, and to keep the cross modulation of high A.F. wave components at a low Value, it is necessary that phase delay in the I.F. system be kept nearly constant over the I.F. range. Otherwise distortion is introduced by superimposed phase modulation. The use of resistance-coupling in the mixer circuit and throughout the I.F. circuits of the monitor reduces this type of distortion to a minimum. In order to vindicate peak modulation accurately, it is necessary to preserve the exact wave form of the audio signal as it comes from the demodulator. This requires not only low distortion in the A.F. amplifier, but also negligible phase shift of various components of the audio wave with respect to each other. In this instrument, the audio phase delay is such that any c-f wave component is not shifted in phase (with respect to a 1000 cycle component) by more than 8 per cent of its own wave length.

The two types of phase-delay effects just cited are particularly important.` in modulation measurements on F-M transmitters, due to the large 13 amount of pre-emphasis used at high audio Vfrequencies.

The Flasher and Meter switches (S3 and S4, respectively) permit the monitoring of modulation Apeaks in either the lhigh or low direction, which is equivalent to indicating audio signal peaks of opposite polarity at the amplifier output. This feature is provided because the audio wave peaks of some types of programs are quite unsymmetrical. Orchestral music will, in general, produce wave forms having peaks approximately equal in rmagnitude on both sides of the reference line. Some speakers, vocalists, and solo instruments may produce quite unsymmetrical wave forms, and the resultant direction of maximum modulation or will depend upon the microphone polarity and all line connections to the transmitter. rIwo people in dialogue may produce maximum peaks in opposite directions if they are on opposite sides of certain microphones. It is desirable to monitor the side of the audio signal producing the greater number of maximum peaks.

I claim:

1. In apparatus for analyzing a wave length modulated wave, a wave amplitude limiter excited by -said modulated wave, a wave frequency detector coupled to said wave limiter and excited by said amplitude limited Wave length modulated wave for 4deriving a potential Ysubstantially proportional to the wave frequency, a tube system coupled to said detector and excited by said derived potential, said tube system having two outputs at which substantially identical potentials corresponding -to said rst potential but of opposed phase appear, a normally extinguished glow tube, and connections for coupling said glow tube to either of said outputs.

2. In a system for monitoring wave length modulated wave energy, va vcurrent amplitude limiter having an input excited by said wave length modulated wave energy, said amplitude limiter having an output, a'wave cycle counter circuit having an input coupled to the output of said current amplitude limiter, said wave cycle counter having an output wherein a potential proportional to the wave frequency appears, a iiasher tube, two paths arranged to alternatively couple said asher tube to the output of said cycle counter, and a in one of said paths.

3. In a system for analyzing a, wave length modulated wave, a wave length detector having an input excited by said wave length modulated wave, said wave length detector having an output, an electron discharge tube having an input coupled to the output of said detector and having potential polarity reverserV an output impedance connected to its anode and an output impedance connected to its cathode, a peak potential indicator and a current meter and a switch for coupling said meter and said indicator to either of said output impedances.

4. In a system for analyzing a wave length modulated wave, a wave length detector having an input excited by said wave length modulated wave, said wave length detector having an output, a low pass :lter having an input coupled to the output of said detector, said low pass filter having an output, an electron discharge tube having an input coupled to the output of said low pass lter and having an output impedance connected to its anode and an output impedance connected to its cathode, a peak potential indicator and a current meter and switching means radiante 121 for coupling said meter and said 'indicator to either of said output impedances.

In a system for analyzing a wave length modulated wave, a current amplitude limiter excited by saidinodulated wave, a wave length detector having an input coupled to said current amplitude limiter, said wave length detector having an output, allow pass iilter having an input coupled to the outputof said detector, an electron discharge tube having an input coupled to the output of said low pass lter and having an output impedance connected to its anode and an output impedance connected to its cathode, a peak potential indicator and a current meter and switches for coupling said meter and said indicator to either of said output impedances.

6. In apparatus for producing indications of peaks of recurring potentials, a glow tube, an electron discharge device having a cathode, an electron receiving electrode and a control grid, a relay having a winding and normally closed contacts arranged to be opened by current now through said winding, a source of potential, a relay operating circuit including in series said glow tube, contacts of said relay, said source of potential, the winding Yof said relay and the impedance between the cathode and electron receiving electrode oi said device, and connections for applying said recurring potentials to the control grid of said discharge device to alter the conductivity thereof in the presence of peaks of said `recurring potentials.

7. In a frequency modulated wave monitor, a wave frequency detector for deriving, from a `modulated wave, potentials proportional to the frequency thereof, a potential peak indicator, an electron discharge device having output electrodes Vand a control grid, a source of potential and a relay having contacts and a winding for controlling the position thereof, a circuit including said potential peak indicator, the contacts of said relay, said source of potential, the winding of said relay and the output electrodes of said device, and a coupling between said detector and the control grid 4of said device for controlling the conductivity thereof in accordance with the derived potentials.

8. In a frequency modulated wave monitor, a wave frequency detector for deriving, from a modulated wave potentials proportional to the frequency thereof, a glow tube having an energizing circuit including the contacts of a relay, a source of potential, the winding of Asaid relay and the impedance of a in series, and a coupling between said detector and the control grid of said gas tube for controlling the conductivity thereof in accordance with the derived potentials.

9. In a system for monitoring wave length modulated wave energy, a current amplitude limi-ter having an input excited by said wave length modulated wave energy, said amplitude limiter having an output, a wave cycle counter circuit having an input coupled to the output of said current amplitude limiter, said wave cycle counter having an output wherein a potential proportional to the wave frequency appears, a current meter, two paths arranged to alternatively oouple said current meter to the output of said cycle counter, and a potential polarity reverser in one of said paths.

10. In a system for analyzing a wave length modulated wave, a wave length detector having an input excited by said wave length modulatedwave, said wave length detector having an outgrid controlled gas tube v put, an electron discharge tube having an input coupled to the output of said detector and having an output impedance connected to its anode, and an output impedance connected. to its cathode, a peak potential indicator and a switch for coupling said indicator to either of said output impedances.

l1. In a system for analyzing a wave length modulated wave, a wave length detector having an input excited by said wave length modulated wave, said wave length detector having an output, a variable gain amplier having an input coupled to said detector output, said variable gain ampliiier having an output, a potential peak indicator, two paths arranged to alternatively couple said variable gain amplier output to said peak indicator, and a potential phase reverser in one of said paths.

l2. In a system for analyzing a wave length modulated wave, a wave length detector having an input excited by said wave length modulated wave, said wave length detector having an output, a variable gain ampliiier coupled to said detector output, said variable gain amplifier having an output, a Acurrent meter and a potential peak indicator, two paths arranged to alternatively couple said variable gain amplier to said meter and to said peak indicator, and a potential phase reverser in one of said paths.

i3. In a frequency modulated wave monitor, a -wave frequency detector for deriving rfrom a modulated wave potentials proportional to the frequency thereof, an electron discharge device having output electrodes and a control grid, a source of potential and a relay having at least two pairs of contacts and a winding for controlling the said contact-s, a circuit including a pair of said contacts of said relay, said source of potential, the winding of said relay and the output electrodes of said device, a coupling between said detector 'and a control grid of said device for controlling the conductivity therein in accordance 16 with the derived potentials, and connections to the other pair of said contacts to which an alarm or'warning device may be connected.

lil. In apparatus for producing indications of peaks oi recurring potentials, a glow tube, an electron discharge device having a cathode, an electron receiving electrode, and a control grid,

a relay having a winding and contacts for completing at least two circuits, the said contacts being controlled by the current through said winding, a source of potential, a relay operating circuit including in series said glow tube, contac.,s of said relay, said source of potential, the winding of said relay and the impedance between the cathode and electron receiving electrode of said device, connections for applying said recurring potentials to the control grid oi said device to alter the conductivity thereof in the presence of peaks of said recurring potentials, and means for coupling an alarm circuit to other contacts or said relay.

15. Apparatus as recited in claim le, wherein a resistance is connected in shunt to said glow tube.

JOHN M. BRUMBAUGH.

REFERENCES CITED The following references are of record in the iile of this patent:

UNITED STATES PATENTS Number Name Date 2,218,923 Newhouse Oct. 22, 1940 2,272,768 Crosby Feb. l0, 1942 2,276,672 Roberts Mar. 17, 1942 1,616,156 Vroom Febjl, 1927 1,831,744 Laurent 1 Nov. 10, 1931 1,933,976 Hanson Nov. 7, 1933 2,309,481 Summerhayes Jan. 25, 1943 1,733,045 Baker i oet. 22, 1929 2,188,159 Rockwood Jan. 23, 1940 

